The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Engine operation involves combustion that generates exhaust gas. During combustion, an air/fuel mixture is delivered through an intake valve to cylinders and is combusted therein. After combustion, the piston forces the exhaust gas in the cylinders into an exhaust system. The exhaust gas may contain emissions such as oxides of nitrogen (NOx) and carbon monoxide (CO).
An exhaust treatment system is typically used to reduce vehicle emissions. A control module monitors engine operation and a dosing system selectively injects urea into the exhaust upstream from a catalyst. The treated exhaust reacts with the catalyst. A chemical reaction occurs that reduces emissions in the exhaust. More specifically, the chemical reaction decomposes the NOx and urea into separate elements. The urea decomposes into ammonia, which is stored on the surface of the catalyst. The ammonia stored on the catalyst reacts with NOx resulting in the formation of nitrogen and water. During cold-start periods, the exhaust temperatures are relatively low. NOx that is formed during the combustion process reacts with the ammonia that was previously stored on the catalyst surface. As a result, ammonium nitrite is deposited on the surface of the catalyst. The formation of ammonium nitrite occurs when ammonia exists on the catalyst surface and exhaust temperatures are below 60 degrees Celsius (60° C.). Ammonium nitrite is stable below 60° C. As the temperature of ammonium nitrite increases above 60° C., the compound decomposes. The decomposition of ammonium nitrite is highly exothermic and tends to increase catalyst temperatures.
The dosing system typically includes a urea supply and an injector. The urea delivered should be based on the level of emissions to effectively treat the exhaust gas. An insufficient treatment of the exhaust gas may be caused by a faulty injector or an insufficient supply of urea.